Environmental protection statutes and regulations are beginning to place limits on the combustion products of small power tools, such as gasoline powered lawn mowers, edgers, chain saws, and line trimmers. Such powered tools usually employ a diaphragm carburetor to control the air/fuel mixture fed to the two-cycle gasoline engine. Fuel is fed to an engine intake path by a fuel pump. The fuel pressure is regulated at a fixed pressure by a fuel pressure regulator. The fuel pressure regulator is equipped with a fuel chamber that stores fuel sent from the fuel pump. A diaphragm that forms one of the fuel chamber walls and a control valve that is interlocked to the motion of the diaphragm opens and closes the fuel chamber inlet. Thus, in any position, fuel is properly supplied to the engine. Fuel travels to the intake path from the fuel chamber through either a main fuel path or an idle fuel path.
A manual fuel mixture adjustment screw is provided for independently controlling the effective areas of the main and idle fuel paths. The adjustment screw includes a needle-shaped valve that can be advanced into or withdrawn from the fuel path. An engine specialist or technician inspects and adjusts each product before shipment to obtain the optimum air/fuel mixture. However, once adjusted, vibration of the power tool can cause the adjustment screw to change from the selected position. Additionally, the power tool user may adjust the adjustment screw's position in an attempt to produce a desired engine output. However, an improper air/fuel ratio resulting from an improperly positioned adjustment screw can produce undesired exhaust output and degrade the performance of the power tool.
Several devices have been developed in response to the need to maintain the adjustment screw in a predetermined position and to limit user-adjustment of the adjustment screw. For example, U.S. Pat. No. 3,618,906 discloses a "Device and Process for Limiting Idling Fuel in Carburetors." The device comprises a polyamide cap that fits tightly over the head of the idling needle. A vane on the cap abuts a boss on the carburetor body to prevent needle rotation in one direction. Internal teeth of the cap fit over the idling needle. A decrease in idling fuel can be effected by rotating the cap and needle in the direction moving the vane away from the boss.
Soviet Patent No. 1,198,235 discloses a carburetor idling system regulator. A regulating needle is threaded into a housing. A bush is attached to the projecting end of the needle and has a radial projection which contacts either front or rear projections on the housing. The front and rear projections limit the rotation of the regulating needle but do not eliminate post-adjustment rotation.
U.S. Pat. No. 5,236,634 provides a pair of needle valves to adjust the main and idle fuel mixture, respectively. A cap is secured to the shank of each needle valve at any one of a series of positions. The caps include radial projections whereby rotation of one cap is limited by engagement of the radial projection with the other cap or with a projection from the carburetor housing or engine.
U.S. Pat. No. 5,055,238 discloses needle valve adjustment screw caps which include radial projections that engage fixed stops on the carburetor body to limit adjustment of the needle valve adjustment screw.
A need exists for a device which can be attached to the adjustment screw of a carburetor. The device must limit the amount of any post-adjustment rotation of the adjustment screw available to the user. However, even though easily installed at the factory, the device must not be easily removed by the end-user.